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What are the rules for an IFR departure from a private airfield with nearby obstructions?
I'm flying out of a small private coastal airfield in California that has a variety of obstructions including nearby trees and hills to the North and East of it, and am trying to understand the legal minimums for IFR departure. There are no other nearby airports or airspaces.
When I call center on the phone they'll give me a clearance "climb and maintain seven thousand, contact center on..." and the release time and void time, but I'm not clear on the legal visibility and cloud clearance minimums for the climb-out and up to the cleared altitude. Must I remain clear of clouds while in Golf? What about upon entering Echo during the climb, but before reaching the assigned altitude?
I'd generally file and be cleared direct (which would be East), but there is terrain to the East so the local process whether VFR or not would be to depart the NorthWest facing runway, then at the end of the runway, turn left towards the ocean, and then continue turning left to on course direct.
Thanks for any legal IFR insights.

Related: https://aviation.stackexchange.com/q/92387/34686, https://aviation.stackexchange.com/q/38842/34686 (question is obviously different but some answers bear on your question), https://aviation.stackexchange.com/q/21888/34686, https://aviation.stackexchange.com/q/47569/34686, https://aviation.stackexchange.com/q/92387/34686.  Your question may be a duplicate of one of these but I'm not certain --

See also https://aviation.stackexchange.com/q/79262/34686 and https://aviation.stackexchange.com/a/45324/34686

Two things are at play here, when your IFR clearance takes effect and when the FAA assumes responsibility for your terrain and obstruction clearance.
If your clearance is "Cleared to XYZ as filed, climb and maintain seven thousand" without any further qualifiers, then you are cleared IFR. Full stop, end of story, that is an IFR clearance. In order to depart into Class E you also need to obtain an IFR release, but assuming you have both the clearance and the release you are perfectly legal to go blasting off from the airport, even in IMC, in fact even in 0/0 conditions (at least under Part 91 rules).
ATC is allowed to issue IFR clearances that pass through uncontrolled airspace (see, e.g., JO 7110.65 4–3–2b1, "Specify the destination airport [as the clearance limit] when practicable, even though it is outside controlled airspace.") Once you have your IFR release ATC will restrict IFR departures out of and arrivals in to your airport—although it is possible another aircraft may be operating IFR without a clearance, unless there is a Class E surface area. ATC cannot and will not guarantee IFR traffic separation in uncontrolled airspace, even if your clearance takes you through such airspace.
To sum it up: Provided you are properly certificated and current and everything else, you are permitted to fly IFR and also to enter IMC in Class G airspace immediately upon departure, whether or not you have received an IFR clearance. In order to fly IFR in Class E airspace (whether IMC or VMC) you need to receive a clearance, which you have in fact received. Everything about your scenario is perfectly legal as far as that goes.

As for terrain and obstruction clearance, if and only if your airport has an official Instrument Approach Procedure, then you can look it up in the Terminal Procedures booklet and see what (Obstacle) Departure Procedures it has. Each runway will have an (O)DP listed, or it will be listed as "N/A" (not authorized) for some reason. In the absence of either one, the standard departure procedure applies: cross the departure end of the runway at least 35' AGL, then continue to 400' AGL before starting a turn, and then continue climb at a rate of 200' per nautical mile or greater up to the minimum IFR altitude.
If your airport does not have any official IAP, then nobody has surveyed it to see what, if any, (O)DP is necessary. Instead you are the final authority as to whether a departure into IMC is safe, and what sort of procedure you would need to fly in order to ensure obstacle clearance. See this question already linked by quietflyer and my answer to it for further discussion.
ATC's rules do not distinguish between a Class G airspace that has been surveyed for instrument departures and one that has not. The phraseology should be the same in either case. It is entirely the pilot's responsibility to know the difference and how it affects your ability to safely depart into IMC.
Once they grant your IFR release ATC will be "blocking" the airspace in the immediate vicinity of the airport up to the initial altitude in your clearance. This means they are protecting that whole area so when you pop up they don't have an immediate loss of separation with another IFR aircraft nearby. This also means you have a little bit of wiggle room to maneuver before turning directly East in accordance with your clearance, but telling them what your plan is would probably be appreciated.

Note: This situation is a little bit muddied because controlled airspace extends so low in so much of the country. There are stories of pilots in past decades flying IFR, and IMC, on cross-country journeys entirely in Class G and without a clearance—legally. Today it would be extremely difficult if not impossible to comply with minimum IFR altitude rules while remaining in Class G. This also means that ATC de facto does provide separation to IFR aircraft as they land and depart Class G airports, even though the letter of the law means such separation is not guaranteed to be complete.

This is probably among the top 10 answers I've seen on Aviation SE.  Personally though, I have always struggled with the concept of IFR without a clearance in G.  It seems that violates the "R" in IFR.  That you are more accurately described as flying in IMC in the absence of a requirement to operate under IFR.  Or is there an actual "rule" under IFR that allows this?

@Michael I don't think there's a specific "rule" which allows IFR in G; rather there is a rule which prohibits IFR in E without a clearance (91.173). The FARs do explicitly account for the possibility of IFR in G, eg 91.179(b) "each person operating an aircraft under IFR in level cruising flight in uncontrolled airspace shall maintain an appropriate altitude as follows..." Other IFR "rules" are written as airspace-agnostic include 91.167, 91.171, 91.177, 91.179, etc.

In accordance with ICAO Annex 11 Appendix 4, IFR flights in Class F/G are not subject to an ATC clearance. No rule requires a clearance in order to fly under IFR in such airspace, just like there is no rule requiring a clearance in order to fly VFR in Class E.

Thanks.  I'm pondering a new question...

There are no legal minimums, assuming you are operating under part 91. If you want to take off under 0-0 conditions, it might not be prudent but it's legal. You're responsible for your own traffic separation and obstacle avoidance in class G airspace. Once you enter class E airspace your route is given non-radar traffic separation until you get into radio contact.
Traffic separation is unlikely to be a problem in IMC. Prolonged legal IFR flight in class G is not possible in California so the traffic separation you are provided in class E essentially protects your departure as well. It's not really possible to protect class E without essentially protecting class G by default.
Obstacles are a problem. To be safe it may be wise to only depart when you can avoid all the obstacles visually. Making your own pseudo-ODP is legal but make 100% sure you know what you're doing and that it provides accpetable clearance regardless of weather conditions. Don't zoom in too close- you should go the extra mile and make sure your flight path will be clear in any weather conditions you'd fly in all the way up to the OROCA.

I assume you are referring to a part 91 operation.

I am surprised about the clearance you got, are you sure you are not missing a part that says "upon entering controlled airspace, this-and-that"? ATC is not allowed to clear any aircraft into G. It is uncontrolled airspace, and they don't provide separation in G, the main task of ATC for IFR. The clearance they give you may cause you to fly through G, but you are on your own for that part.
The E part should be clear, you have a clearance for the controlled airspace part, you can fly in 0 vis. The tricky part is only the G part.
My flight instructor summarized the question of whether or not to violate vis minimums in G as "legalized scud-running". Realize that scud-running is still clear out of clouds. Flying in the clouds is not scud-running anymore! You are in the clag!
you can fly in G clear of clouds, VFR and IFR. You can not fly in 0 vis, VFR or IFR.
Realize that 0 vis is not just "in the clouds". It also includes dense fog. You can be outside of the clouds and can still be in IMC, for example when in dense fog. Conceivably heavy rain can give you 0 vis. Conceivably heavy rain makes it impossible for you to look out the windshield because all you see is water. That too is instrument meteorological condition! You are in IMC whenever you can't see anything outside and need to fly on instruments, no matter the reason(s) for that. Therefore you should really look at this as IMC, not just "clear of clouds". In the clouds is sufficient for "no", clear of clouds is not sufficient for "yes". Clear of clouds would be one of several necessary conditions for "yes".
you can only fly in IMC, regardless of airspace, if you have a clearance. ATC does not give a clearance for G. That answers your "Must I remain clear of clouds while in Golf" part, which I think is the center of your question.
I think we can also approach this with another purely logical argument. Jurists like hypotheticals, and I think this is another way to see this. If, hypothetically, flying in the clouds in G were legal, then two planes could be in the same cloud under the same flight rules, and don't see, hear, or know of each other (possibly not on freq, possibly no xpndr, possibly no ADSB, these are simply not required in G). That alone cannot be allowed (two in the same cloud not knowing of each other), therefore flight through the clouds in G is illegal. At the very least it would be careless / reckless.

I think that should cover the legalities, which is what you have asked for. My recommendation (although you didn't ask for it):

Already flying clear of clouds under basic VMC in G is playing Russian roulette, so I wouldn't do it, a) for safety reasons, and b) it may also be construed as careless or reckless, 91.13. Someone else may be doing the same thing and you don't see, hear, or know of each other, and the other guy may be "just around the corner" of that cloud in G, legally there, just like you, and kaboom. I'd fly close to the clouds only in positively controlled airspace, and E and G are not positively controlled, G is even uncontrolled.
There are reasons commercial operations are not allowed in G under IFR. And you'll remember from back when you did your CPL that pilots were encouraged to embrace part 135 standards even for part 91 flights. That alone is a no-no for me for many scenarios that would be legal under part 91. If you're allowed to do it, someone else is allowed to do it, and kaboom. That's why ATC will never clear you into or though G. There are several other things you are allowed under 91 you are not allowed under 135 or 121, for example taking off in 0-0. That means you would be allowed to take off on an airport that you would not be allowed to return to if you developed engine problems immediately after take-off. Whatchagonna do now? Or you don't need a functioning landing light under part 91. But is that smart? Bottom line: if you can't do it under 135, don't do it under 91. Remember: the life you save may be your own. Life is precious, especially when you fly a Mooney :) Think about all the many Mooney hours you can still have when you don't mess with clouds in G.

If you are IFR, you can legally fly in 0 vis even in class G. Whether this is prudent is a different question.

Several things are incorrect about this answer... ATC can and will issue an IFR clearance both into and out of Class G airspace. And you do not require an IFR clearance to fly under Instrument Flight Rules (including IMC) in uncontrolled (Class G/F) airspace.


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How much does the wing of a 6-seater plane weigh?
I could find plenty of information regarding the total weight (empty operating weight, maximum landing weight, and maximum takeoff weight) of every plane in commercial production. However, I could not find information regarding the weight of the wings, fuselage, or empennage of a small airplane. So, how much do the wings, fuselage, and wings each weigh for a 6-seater front-engine airplane such as a Cessna 206 or Cherokee Piper Saratoga?
Knowing this would be useful if an upper middle class family wants to take the used airplane on a 2-week cross-country trip four times per year and they want to save on hangar rental costs because it is only used during that time period. Upon arriving at each destination, they rent a car for a day or two. They bought a plane rather than the much cheaper option of taking the airline because they never want to take public transportation while not having to waste 10 hours driving on the highway between each leg, and that renting a plane four times per years for 2 weeks at a time costs more. So, for a family of four (with the 3rd row being extra luggage space), how feasible would it be for 1 person to bolt and unbolt the wings of a 6-seater airplane with the remaining 3 holding up the wing during the process so that they can tow the plane on the street to and from the camper and boat parking space located on the side yard of the home?
I have no idea how much airplane parking space rent at a county airport costs. Is it as expensive as car parking rates in downtown garages, given that aviation is generally an upper class hobby?

You do realize that the wings is where the fuel tanks are, right? Removal is FAR more than a couple of bolts.

I thought the wings were the fuel tanks only for large airliners because giant planes are very heavy. Also, unbolting and bolting could each take half a day in this case, because the time spent would more than be saved compared to driving the entire multi-thousand-mile trip on the highway in a car.

See this for a Cessna 206: https://www.aero-news.net/images/content/genav/2010/Sierra-Soloy-FuelTankMods-0410a.jpg Note the darker grey for "Original fuel tanks"

In addition to the fuel tank issue, you also have the linkages for the control surfaces. The wing is not just a simple slab bolted on to the side. This would need a certified A&P mechanic, and inspection. You need to rethink this.

Will you be paying a qualified person to do this work? You can't do it yourself you know. That labor cost will blow any savings from not renting a hangar away.

A pilot who only flies four times a year is not a safe pilot.  https://simpleflying.com/pilots-minimum-hours-per-year-guide/

Some small planes (2 seaters) are designed with wings that fold back.  Google "Series 7 Kitfox" -- or see https://kitfoxaircraft.com/aircraft-kits/#

As others have shared, it is not plausible to save money in this way.  Aircraft have lots of fixed operating costs -- insurance, annual inspection, depreciation, interest, etc.  (and hangar rental)  Things you must pay whether you fly it or not.
As mentioned, removal and replacement of the wings is a major maintenance activity that would require a certified A&P mechanic.  The process would not be cost effective.
The way to make an aircraft affordable is to increase utilization.  Fly the wings off of it.  Amortize the fixed costs over many operating hours.
In this realm, your family that only wants to use an aircraft a few times a year would be much better off either renting or at least in a shared ownership arrangement.
Owning your own aircraft (and not flying it much) is not going to be cheaper than renting or shared ownership.

I didn't know aircraft can have shared ownership too. I only know boat rental companies rip off customers because those who use them do so for vacation. So, I thought it would be the same too for airplanes, in which buying the plane costs just 3 months' rent for example.

Now shared ownership makes the most sense because the cost really is shared because no one is profiting. That is unlike a landlord and tenant situation, where the tenant is a wage slave to the landlord in which the landlord just gets wealthier and wealthier, especially inherited over multiple generations, without doing anything.

Without getting into politics of real estate, renting cars, construction equipment, tools, and airplanes is really a different thing.  In a shared ownership situation, if the engine fails (or other major maintenance cost comes up), then all the partners in the aircraft must come up with the cash to get the asset operational again.  When you rent an aircraft, you pay your hourly cost and walk away after the flight.  The owner will (and should) add some amortized maintenance expense into the hourly rate -- but as a renter, you'll never need to face a 5 or 6 figure engine overhaul bill.

There are statistical methods for determining the component masses of airplanes. See for example the books of Jan Roskam or Egbert Torenbeek. Usually, those methods require parameters like wing span, wing area, maximum load factor and take-off mass, and they differentiate between cantilever and braced wings. For faster airplanes, additional information like the design dive speed, wing sweep angle and taper ratio help to get more accurate results. When applied correctly, these methods easily achieve errors below 10%.
As an example, here is the Cessna method for cantilever wings of light GA airplanes (maximum speed below 200 kts):
$$m_{Wing} = 0.04674\cdot m^{0.397}_{Take-off}\cdot S^{0.36}_{ref}\cdot n_{z_{max}}^{0.397}\cdot\left(\frac{b^2}{S_{ref}}\right)^{1.712}$$
As you can see, this is a numeric value equation, so units are important. This one uses only feet and pounds.
As the others have explained already, taking the wing off to save a few days of hangaring will not be cost effective. You can do this with gliders (which nowadays use automatic connections for the control linkages), but not with GA airplanes unless they are specifically designed to make this possible. That I give a direct answer is mostly to benefit future readers who are looking for ways to determine estimates of component masses.

Wings aren't very heavy. In principle, an aircraft could be specifically designed for this to be a relatively straightforward operation, as it is for most gliders. Some light airplanes even have wings that fold up, though I don't know if there are any with six seats.
It's still a terrible idea. Parking costs aren't the only fixed costs or even that large of one. An annual inspection alone for a 206 is in the ballpark of $3000 even if there are no problems found. And in an aircraft that is left to sit for months at a time there are very likely to be problems.

As a rule of thumb, statistically the wing of GA aircraft weighs 12kg per m² of surface.
The 16.30m² big wing of a Cessna 206 should therefore weigh a bit less than 200kg.
More advanced methods take into account the real geometry of the wing and the performance of the airplane, but as a starting point that value should be enough.
Feasibility and cost of your idea has been already covered by the other answers.

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Is it realistic that a plane reacts very quickly to rudder input as shown in a MSFS video?
Microsoft Flight Simulator is supposed to be a "simulator", so following the laws of physics.
However, I'm looking at a gameplay video (at 2:17) and it shows a plane reacting really quickly to the rudder.
By my estimate (I was a physics major), this does not seem realistic (consider the Gs the front and tail must be experiencing). But on the other hand I don't understand why Microsoft would compromise their own game. So I guess the two disbeliefs cancel each other somewhat.
Is the behavior of the plane shown in the video clip realistic?

The short of it is yes, but not for the Airbus A320 shown. The control surfaces can initiate that sort of violent movement, the flight computer simply will not allow such command to reach the actuators.

@PcMan The rudder on the A320 is not fly-by-wire. The rudder pedals are mechanically connected. The only restriction from the flight control computers (the FAC in this case) is the rudder limit as explained in ymb1's answer. See also this answer for a description of the A320 rudder.

MSFS is rather-well-known for its... quirky physics.

That is a very rapid and instantaneous motion indeed.
Yes MSFS is a "simulator" - game. Its main selling point is visual reproduction on a PC screen of aircraft instrumentation and out-of-the-window views, and it can help in learning what aircraft systems there are and how to operate them. But how exactly the instruments respond is of secondary concern.
In contrast to this is a Level D Full Flight Simulator, which must demonstrate to aviation authorities that simulated aircraft dynamics are within tight tolerances of flight test data from the aircraft. For instance the sideslip angle response to a pedal input with both Yaw Damper ON and Yaw Damper OFF.

Responses above would be for a Level D sim of a subsonic passenger jet with gear UP, yaw damper OFF, at 10,000 ft and about 160 kts.

The pedal is deflected to about 1.5° in 2 seconds - a finite time, there is damping in the pedal feel, plus an opposing force.
The rudder deflects much in unity with the pedal, actuated by the hydraulic system.
As a result of the rudder deflection, sideslip angle enters a second order response with inertia and damping effects.

Peak value of $\beta$ = 2.4° is reached after 6 seconds, which is indeed not what we see in the video. Plus the response is an acceleration, as can be seen from the gradual increase of $\beta$ starting from zero until a constant yaw rate is reached.
So is the rapid and instantaneous MSFS response realistic? Not really, no. It looks like a rudder deflection equates to an immediate yaw rate, there is room for improvement.

"there is room for improvement" I must say I don't understand why MS didn't get this right, considering that they do have an interest in making the game as realistic as possible.

I fully agree with you. It gets even worse with some helicopter responses that I've seen which dive like a fixed wing when moving the stick forward, instead of mainly picking up airspeed.

@ymb1 They are code generated, but the code resides in a Full Flight Sim with pedals loaded according to the real aircraft, requiring a high force to deflect fully when flying. Recording and matching of responses is done only for circumstances commonly encountered during flight, such a full pedal manoeuvre is not part of that sub-set. But the response to the game manoeuvre would be similar to above: a sine-wave like return and overshoot, continuing for several periods. With a longer period than above since vertical tail damping is higher, inertia stays the same.

Before anyone asks about the waviness in the rudder-angle graph, that's presumably due to the variation in the aerodynamic forces on the rudder (and, thus, variation in the rudder's blowdown limit) with sideslip angle.

@Vikki Indeed, combined with the elasticity and friction in the linkage between actuator and the rudder, plus the hydraulic compressibility of the actuator (oil is compressible)

Is it realistic that a plane reacts very quickly to rudder input ... ?

It depends on the plane and how much rotational inertia it has in the yaw axis, in comparison to the aerodynamic yaw torque generated by the rudder.  In something like a Cessna 152, or a Schweizer 1-26 sailplane, the yaw response to rudder input is pretty quick.  Yet even in these aircraft, if the pilot gives alternating left and right rudder inputs, timed in synch with the aircraft's yaw response to the previous input, he or she can "pump up" a yaw oscillation that achieves a significantly larger maximum slip angle (as indicated by the displacement of the slip-skid ball, or the yaw string) than would exist if the pilot simply held one rudder pedal all the way down.  (Think of a kid on a swing, reaching a higher altitude with each arc.)  This is an indication that even in these lightweight aircraft, yaw rotational inertia is not trivial, which means that the yaw response to a rudder input cannot be instantaneous.
In an airliner, naturally, these dynamics would be much more pronounced.

I've got video stashed away on some hard drive somewhere, of a hang glider pilot using the alternating left-right adverse yaw torques resulting from a series of alternating left-right weight-shift roll inputs, to "pump up" a yaw oscillation involving sideslip angles vastly larger than could otherwise be obtained in this rudderless aircraft-- again this would not be possible in an aircraft with negligible rotational inertia in the yaw axis--

Yeah if you hit the eigenfrequency in yaw you’ll be achieving ever higher amplitudes. But is the response to a step input physically realistic in the video, I reckon it isn’t according to physics.

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Portable VTOL Startup
I am Alessandro and I am currently developing a new portable electric VTOL that can be carried by car without a trailer and allows a flight time of around 15 min carrying one person.
The expected price will be around 70k.
May I ask you for some feedbacks about this idea?
For example, if you would be more interested in a lower price with reduced flight time, or your potential use case or habits if there are.
Thank you

Just 15 mins of flight time seems pretty short imo. How do you intend to transport without a trailer?

I wouldn't bother taking it out of the trunk for 15 minutes of flight.  Much less drop $70k for the privilege.  If i wanted to take an aerial photo of my car I'd buy a small drone.  Sorry to be blunt, but that's my feedback.

Please don't be disheartened that your question was closed. If possible can you edit it to included a more focused question? Then you can request it to be reopened.

What kind of VTOL do you have in mind exactly?

For that price and that short amount of flight is a little bit pricey. I can't think of any mission that short I would do. Only occurs to me something like getting to a beautiful place in your car but there is no more road access and you want to see more of that place and get a nice air view. It would be like having a DJI drone that you can ride. For that I would pay maybe max 50k.

15 minutes is not very long in flying, and that level of endurance would only be acceptable for a motorglider.
You want an hour at minimum, and that still only makes it an amusement device. Electric paramotors just get there, mainly thanks to extremely low wing loadings so little energy is required to sustain flight.
I'll provide an opinion, but based on a fundamental fact.  Electric flight is going nowhere with batteries that can only manage 250-300 Wh/Kg, and can randomly turn into roman candles (Boeing learned this the hard way with the 787's batteries, being forced to design L-ion-fireproof enclosures to get the fleet back in the air).
When a battery comes along, that with motor can match my aircraft engine and fuel supply pound for pound while providing the 4 hours endurance that gas does, while being immune to spontaneous self-discharge, I'll convert my own plane to electric.


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Computing the eigenvalues of the longitudinal dynamics _analytically_: is there a method to simplify the task, or must it be brute force?
I understand how to go about finding the eigenvalues and eigenvectors in principle, in order to distinguish the phugoid modes, but the first exercise in Lavretsky & Wise tells to compute those values analytically.  All examples online first pick numerical values and then compute the eigenvalue, as numerical values.  Am I misunderstanding the problem in Lavretsky & Wise, or are they telling to express the eigenvalues algebraically in terms of the entries of the general matrix of longitudinal dynamics?  And if they are, are there some observations that simplify the task, or must one plough through the roots of a quartic polynomial brute force?

This seems like it may be better suited to Math.SE instead.

I will ask there, thank you. I asked here because I thought, perhaps, there is some engineering intuition here.

Also posted on Math.SE: https://math.stackexchange.com/q/4861343/174736

@AndrewT., yes, by me.:)

are they telling to express the eigenvalues algebraically in terms of the entries of the general matrix of longitudinal dynamics?

The way I read the problem yes they are.  Although I'm not familiar with this book, what they are probably trying to get you to see is some relationships between the variables.  E.g. if you solve this numerically, you've got to pick values for all of the variables.  You have to pick an airspeed V0, turn the crank, and an eigenvalue will pop out.  Now ask yourself, how does increasing airspeed (or some other variable) affect the eigevalues?  Well you have no idea.  You have pick a whole bunch of other values of V0, compute a whole bunch more eigenvalues, make some plots, and then try to discern if its is a linear relationship, quadratic, etc.  But if you solve it analytically, the answer will just pop out.  V0 will be right there in the answer, and you can see exactly how it affects each mode.

And if they are, are there some observations that simplify the task,

Well you've got a bunch of zeros in the bottom row.  That could be of great help in expanding the determinate.


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EASA Medical Centre Canada
I am looking to go to Europe for flight school, I need to get a EASA Class 1, are there any EASA approved medical center's in Canada?

There are CAMEs in Canada who are licensed to do FAA medicals, but I've never heard of one that does EASA ones.

That's an answer I'd upvote @DJClayworth.

I don't know of any EASA medical examiners in Canada. However a class 1 medical can be issued the same day of the exam, so you could book a medical in Europe the day before you are due to start flight training. If you are starting from scratch a class 2 might be sufficient to get you through the initial training. It would probably be a good idea to have a Canadian ME look you over first - they can tell you if there is anything wrong with you that might cause you to fail the EASA class 1, even if they can't give you the EASA certificate. That way you wouldn't waste money on flying to Europe and booking flight training only to fail the medical.


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How does aircraft leasing work?
I've noticed on websites where you can buy and sell aircraft, such as controller.com and trade-a-plane.com, there are often aircraft that are available for lease. For example, there is a 1997 Bonanza available for $4,000 a month on controller.com. I'm wondering if anyone knows anything about how aircraft leasing works. Is it similar to leasing a car? What is generally the minimum amount of time you can lease for? Is the person leasing the aircraft responsible for all maintenance costs, etc. If anyone knows anything about how this works, please let me know.
Thanks!

This question is more of a financial nature than an aviation question. As such it should probably be posted at https://money.stackexchange.com/ instead of here.

Leasing airplanes is a popular way of exploiting certain quirks of the tax laws to (slightly) reduce the costs of owning and operating an airplane. The basic idea is similar to that of leasing a car but the details are more complex, in that the business entity that leases out the airplane experiences certain tax benefits in the transaction.
Those details get worked out between the parties in a process in which their lawyers have copies of the tax code in their laps while they decide exactly who pays for maintenance, etc.
Deals like this only make sense if you are very wealthy and have smart financial advisors working for you. So for example when Bill Hewlett and Dave Packard were both still alive and involved with HP's business, it was said that each personally owned a Rockwell Sabreliner which they leased to HP. HP could then deduct the lease cost as a business expense but because the jets did not show on HP's books as capital assets, HP could not depreciate their value annually. Bill and Dave showed the lease payments as income but got the benefit of the depreciation and could reserve the use of either jet for their own use at will. The net result was sufficiently beneficial to both H & P and to HP that they managed their two-jet fleet in that way.
Isn't capitalism grand?


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Do constant speed propellers actually increase fuel efficiency?
I have a few questions regarding constant speed props. As a pilot who just recently moved up to aircraft equipped with constant speed props (Cessna 182, Piper Dakota), I understand that a constant speed prop is meant to increase efficiency and decrease engine wear. However, the the performance tables in the POH of the C182T do not support the idea that a constant speed prop results in higher fuel efficiency. For example, at 3100 LBS, standard temperature, a constant 21 inches of manifold pressure, and a pressure altitude of 8000 ft, the ratio of true airspeed to gallons per hour (which is essentially nautical miles per gallon) barely increases with a decrease in engine RPM. From 2400 RPM to 2000 RPM, this ratio is 11.12, 11.42, 11.55, 11.69, 11.87, respectively. This does not seem like a very big increase in efficiency to me, considering the added complexity of a constant speed prop. A reduction in RPM with a constant manifold pressure also reduces true airspeed, so couldn't the same effect be produced with a fixed pitch propeller simply by reducing power? This would both reduce airspeed and increase fuel efficiency, so this raises the question, what is a constant speed prop actually achieving? What is the most efficient RPM in terms of nautical miles per gallon? Is it simply the lowest RPM? I feel like there's some knowledge I'm missing here.

With a fixed prop, you have to optimize for either climb or cruise. Variable pitch optimizes for both. Within either regime in in isolation, it’s not a significant difference.

With a constant speed prop you are paying for maximum performance across the operating spectrum.
A constant speed prop will have only a marginal efficiency improvement over a fixed pitch prop optimized for the cruise case (a so-called "cruise prop").  Both propellers will be operating fairly close to their optimum blade angle of attack and the overall performance will be similar.
What the constant speed prop gets you is efficiency  across the speed range.  If you want the best performance from a fixed pitch prop for the climb case, you have to pitch it for that speed range, and now you've lost efficiency for the cruise case. So you have to choose what to give up when you select a fixed pitch propeller.
This goes all the way down to the static (not moving) case.  When stationary, a CS prop will produce substantially more thrust than a fixed pitch one pitched for climb, and way more thrust than a fixed pitch one pitched for cruise.
So in the end it depends on the performance objectives that you are willing to pay for. If a rate of climb of 1000 fpm vs, say, 1200 fpm is not a big deal, and acceleration from a stand still is not that big a deal (because you don't operate from short strips say) I might prefer to go with a fixed pitch prop and save a fortune on maintenance costs and maybe 80 lbs of weight.
On the other hand, if I want to accelerate from a standing start as hard as possible, get the highest climb rate possible, and cruise as fast as possible, all at the same time, then a CS prop is worth it.
The CS prop also gives power setting options you don't have with a fixed prop, like running "oversquare" to minimize RPM, since RPM is the primary determinant of engine wear (it comes down to how far the piston rings travel over a given number of hours), since low RPM is always less heat/friction than high RPM (subject to limitations of the engine itself, where for example, the engine may not be happy below a certain RPM in terms of roughness or vibration).
To answer your question directly, if the comparison is to a fixed pitch prop pitched for cruise, the difference in fuel efficiency will be negligible.  If the comparison is to a climb prop, there should be a measurable difference because the climb prop will force you to run at a higher RPM at cruise to get the same percentage of power.

John K thanks so much. What would be an optimum RPM for say, an IO-540 in terms of efficiency and engine wear?

I would run it as slow as it will run smoothly and use the MP that gives the rated power you are after. Like, say 1900 RPM and 22" if that gives you 60%. There is an old wive's tale that oversquare operation does damage (MP higher than RPM) but it's not true. A Pratt 985 runs at 36" and 2300 RPM at TO power and cruises maybe 1500 and 24" or whatever. Check with the the ops manual for the engine however. There may be MP/RPM settings that create resonance issues with the crank that you are supposed to avoid, not uncommon with sixes with the internal pendulous balance weights on the crank throws.


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Can a private pilot create a Go Fund Me for charitable flights?
I am a private pilot who flies for Pilots N Paws, and I want to create a go fund me to help support my Pilots N Paws flights. I am wondering if this is a violation of a FAR in regards to private pilots accepting compensation. The way I understand it is that the FAA does not want passengers paying unqualified pilots for flights that could potentially put the passengers at risk. In this scenario, there are no passengers, so I would assume it's acceptable for me to accept donations to help pay for these flights, but I just want to make sure. Just to be clear, the money from the go fund me is to help me, the pilot, pay for costs that are normally the responsibility of the pilot, such as crates, pet supplies, overnight hotels, cars, and other expenses associated with Pilots N Paws missions, not directly aircraft rental or fuel costs.

I'm pretty sure that the supplies, pet carriers and other costs for the upkeep and movement of the animals are fine, I think hotels and personal expenses may be a bit shaky.

Pretty positive there’s nothing in the CFRs about being reimbursed for charity work.  It’s definitely a gray area, but I’d advise that you keep meticulous expense records, and maintain as much separation between flight and administrative activities as possible.  (E.g. no pictures of airplanes on your go fund me page, just dogs…)

This is a question I would discuss with my local FSDO.

@GdD - I don’t see why hotels would be problem.  They have nothing to do with flight expenses. They’d exist even if OP drove the dogs instead of flying them.

It is true that hotel expenses would be incurred with driving @Jim, that doesn't mean that it wouldn't be seen as compensation by the FAA. The rules aren't always common sense.

For any expenses you’d incur even if you were driving the pets instead of flying them, there should be no legal concerns at all.
Still, to be safe I’d keep excellent records (including scans/photos of receipts for everything) in case you ever have to prove you’re not violating 61.113.

You might be right, but if it was me I'd want to see some references to back that up.


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Did SWA 997 deviate from the planned stabilized approach to rwy 04 and instead land on 31?
On Monday, January 3, 2022, Southwest Airlines Flight 997 from STL to LGA landed with the following flightpath:

The flightpath deviates from the planned stabilized approach to Runway 04 and instead lands on Runway 31. This involved executing a right turn to an approximate heading of 070 degrees followed immediately by a sharp left turn to 315 degrees to line up with the runway, all within the last 90 seconds of the flight.
Why would this have been done?
To me, an amateur aviation enthusiast, it seems to me this is very much a non-stabilized approach, and I wondered why a commercial airliner would do such a thing.
Is there a place where one could find recordings of the ATC-cockpit communications during this event?
Is this kind of last-minute switch to a perpendicular runway common in commercial aviation?
Is this maneuver indicative that there was a last-minute issue with the planned approach, such as a runway incursion?
Source: https://flightaware.com/live/flight/SWA997/history/20220103/1955Z/KSTL/KLGA


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Does the infinite focal point in a HUD resulting from the collimation mean the HUD is ALWAYS in focus for the pilot?
I've found lots of information regarding inifinite focus/projection, and collimation with HUDs. However, I am unable to get a definitive answer to this question.
I understand that the HUD is in focus for the pilot even if the pilot is looking at a distant object (such as another aircraft in the pattern). The pilot does not have to focus on the combiner glass in order for the HUD to be in focus. But what if the pilot's eyes are focused on the glass right in front of them? Is the HUD still in focus even when focusing on something so close?
If there's a relevant difference, I'm looking for how this would work in a military HUD. Thanks in advance.

The projector and the combiner create a virtual image on the other side of the combiner mirror. This image is perceived like a true image.
The eyes need to adapt the focal length to the distance of the virtual image. If the HUD is set to collimate at infinity, the eyes must accommodate at infinity to have a sharp image. If the eyes accommodate on the combiner frame, the virtual image is blurry. Without entering into the details of how the whole eye works (I ignore them) the overall principle is the focal length must be reduced (lens more convex, refracting more) when the object to accommodate on is closer.
Here is a drawing where the projector is not collimating at infinity, to facilitate the demonstration.

At the top, the eye accommodates on the combiner, it adjusts the focal length to form a sharp image on the retina.

In the middle, the eye, still focused on the combiner, forms a sharp image of the virtual image ahead of the retina, the image on the retina (not shown) is blurred.

At the bottom, to have a sharp image of the virtual image on the retina, the eye refocuses, increasing the focal length, now the combiner image (not shown) is blurred as the focal length is not adequate.

If the projector really collimated at infinity, then:

In the middle, the image of the virtual image would be sharp on the focal plane and still blurred on the retina.

At the bottom, the focal length would be increased

Awesome, this all makes sense and was beginning to be my hunch. Thanks for the explanation!


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Legal Requirement to reweigh aircraft?
For part 91 operations or regular trainer aircraft, what is the "LEGAL" requirement for which an aircraft re-weigh is required?
The ONLY thing I can find on the internet is in the W/B handbook page 9-3 which states: For small cabin aircraft, record a weight change of +/- 1 pound.
This is the first time I have ever heard the phrase "small cabin aircraft" And I am unsure if "record" means re-weigh.

I think you are looking at FAA 8083-1: "Weight and Balance Handbook". It doesn't define small cabin aircraft, but I think the handbook is derived from AC 120-27F, which defines small cabin aircraft as 5-29 passenger seats.
Figure 9-3 talks about the incremental weight change, and the corresponding paragraph says the weight changes are kept in a log, and says changes should be "recorded".
Looking at the AC, page 2-1 says small aircraft need their weight adjusted in the one pound increments. It calls it an incremental weighing, from context that is different than physically weighing the entire plane.
Typically items removed are weighed, then items installed are weighed, and the official W/B chart is updated. In other words, it’s a paper exercise.

That makes plenty of sense! thank you!

If you've bought an older GA aircraft and refurbished it, changing avionics, the interior, maybe adding winglets or extra fuel tanks, you'd re-weigh the whole thing and publish a new W+B chart. I certainly wouldn't trust a series of add/remove calculations based on some averages (that's like trusting calorie counts on a restaurant menu!). And the person signing off on the annual inspection would certainly need to agree that the W+B is an accurate representation of what's in the plane.

@grumpy1arrival is that a legal requirement? That’s went the question asked.


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How are cockpit warning sounds recorded/chosen?
I know that during an emergency, sounds in a cockpit such as "TERRAIN PULL UP" are played or in some planes the stick shaker takes place during a stall. But how are such warning sounds and actions decided and/or recorded?

Like most things in aviation, they started out as something somebody thought was a good idea. Then once you try to put it in an aircraft, the regulatory agency (e.g., FAA) has to review it and put their spin on it. Eventually it gets codified in the regulations or specific equipment standards.
At the aircraft level, the rule is 14 CFR 25.1322 Flight Crew Alerting. It currently reads:

§ 25.1322 Flightcrew alerting.
(a) Flightcrew alerts must:
(1) Provide the flightcrew with the information needed to:
(i) Identify non-normal operation or airplane system conditions, and
(ii) Determine the appropriate actions, if any.
(2) Be readily and easily detectable and intelligible by the
flightcrew under all foreseeable operating conditions, including
conditions where multiple alerts are provided.
(3) Be removed when the alerting condition no longer exists.
(b) Alerts must conform to the following prioritization hierarchy
based on the urgency of flightcrew awareness and response.
(1) Warning: For conditions that require immediate flightcrew
awareness and immediate flightcrew response.
(2) Caution: For conditions that require immediate flightcrew
awareness and subsequent flightcrew response.
(3) Advisory: For conditions that require flightcrew awareness and may
require subsequent flightcrew response.
(c) Warning and caution alerts must:
(1) Be prioritized within each category, when necessary.
(2) Provide timely attention-getting cues through at least two
different senses by a combination of aural, visual, or tactile
indications.
(3) Permit each occurrence of the attention-getting cues required by
paragraph (c)(2) of this section to be acknowledged and suppressed,
unless they are required to be continuous.
(d) The alert function must be designed to minimize the effects of
false and nuisance alerts. In particular, it must be designed to:
(1) Prevent the presentation of an alert that is inappropriate or
unnecessary.
(2) Provide a means to suppress an attention-getting component of an
alert caused by a failure of the alerting function that interferes
with the flightcrew's ability to safely operate the airplane. This
means must not be readily available to the flightcrew so that it could
be operated inadvertently or by habitual reflexive action. When an
alert is suppressed, there must be a clear and unmistakable
annunciation to the flightcrew that the alert has been suppressed.
(e) Visual alert indications must:
(1) Conform to the following color convention:
(i) Red for warning alert indications.
(ii) Amber or yellow for caution alert indications.
(iii) Any color except red or green for advisory alert indications.
(2) Use visual coding techniques, together with other alerting
function elements on the flight deck, to distinguish between warning,
caution, and advisory alert indications, if they are presented on
monochromatic displays that are not capable of conforming to the color
convention in paragraph (e)(1) of this section.
(f) Use of the colors red, amber, and yellow on the flight deck for
functions other than flightcrew alerting must be limited and must not
adversely affect flightcrew alerting.

There is also a companion Advisory Circular, AC 25.1322-1 that provides more detailed guidance.
If you look at the rule, it requires "timely attention-getting cues through at least two different senses by a combination of aural, visual, or tactile indications." In most cases, aural and visual cues are used. Stall warning alerts that use a horn and the stick shaker would be an example of aural and tactile cues.
As the rule states, the alerts must be prioritized based on the urgency: Warning, Caution, and Advisory.
Advisory alerts are the lowest category and don't require immediate action. They typically employ a single "Alert" chime and an advisory message on the Crew Alerting System (CAS) display.
When you get to Caution and Warning alerts, it's important to get the right information to the crew so they can act on it in a timely manner. Generally, there are Master Warning and Master Caution lights along with a distinct chime. For many system faults, you'll get the Master Warning or Caution and an associated CAS message (in Red or Amber).
Some warnings you really need to address quickly. These often use the distinct aural alerts you mentioned. Specifically, there are two kinds of aural alerts; non-verbal and verbal. Most of the earliest alerts use non-verbal aurals; stall warning horn, fire bell, overspeed clacker, a/p disconnect tone, etc. While many are based in the limited technology of the times, they all have two big advantages; they convey the message quickly and they can coexist with other aural alerts. This is important when urgency is paramount.
The voice alerts are more recent and the majority are related to three warning systems: Terrain Alerting and Warning System (TAWS), Windshear alerting, and Aircraft Collision Avoidance System (ACAS or alternately known as TCAS.) The key value of the voice alert for warnings is that it doesn't tell you there's a problem, it tells the crew the immediate action they should take -- "PULL UP! PULL UP!". There are a few voice alerts used for Cautions such as the ACAS "TRAFFIC" alert.
The voice alerts for these systems are defined in the TSO standards for the systems. They have evolved over the years as part of the continuing regulatory process between the regulatory agency and the aircraft and equipment manufacturers.
The one downside to voice alerts is that you can only have one at a time. Having more than one at a time could cause interpretation errors and confusion. So the rule requires strict prioritization of the aural alerts and is subject to regulatory approval.
As to how they are implemented; it's just somebody hired to record the messages which are then digitized and stored in the equipment where the software can call up and play the appropriate message.

"Rule requires strict prioritization of the aural alerts and is subject to regulatory approval."
and still when on approach PIA 8303 landed without gear down... sink rate got more priority than GEAR NOT DOWN.


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Must aviation rental services company do 100-hr inspections?
We rent our airplane to only certificated, rated pilots. These pilots may rent to increase their XC flight time. They may rent to take a family member sightseeing. In some cases they may rent our airplane and then secure the services of a CFI to pursue an advanced rating, like instrument. It is the choice of the pilot renter.
We’ve been doing 100 hr inspections. Must we?

In which jurisdiction are you located?

Oregon. I just saw this response to a previous question:  “If Fly By Knight is going to rent the aircraft to a customer, however, and does not provide the pilot, that aircraft need not have an annual or 100-hour maintenance inspection within the preceding 100 hours of time in service. “


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Effect of Vortex Shedding in lift and drag coefficient?
I want to calculate lift and drag coefficient of a airfoil at multiple using CFD simulation. I am using steady state simulation to run analysis. However I am worried about results. Does steady state simulation captures the vortex shedding effect on lift and drag coefficients?

Are you looking at an airfoil or a wing?

The steady state solver consider that the solution should converge to a steady state. Let's consider for example (and for simplicity) to be in a finite volume solver. What the algorithm is trying to do is to send the residual (generated by the flux assembly) of your Navier-Stokes discretized system to 0.
In the particular case of unsteady phenomena (like the vortex shedding) the residual is related to the time and in your steady problem you are not considering the time dependence (or better it is hidden, if you are interested in it, see the last part of the answer). This would mean that what you will get as a result will be an oscillating field of your states (velocity, density and energy in case of compressible solver) but it is only your solver that is trying to converge to a steady state, with no real physical meaning (at least it depends on the type of the time discretizationa and the way of solving the equation, but this is more a philosophical discussionbut). Furthermore you have to pay attention that you are converging to the correct result. As I told you the phenomenon is unsteady, hence it is not given that the oscillating residual will converge to your tolerance and to the physical admissible solution. For more information have a look at this discussion.
To simplify it imagine that for each iteration your unsteady solver is doing the same, it converge to a steady state and later it advances. Collecting all the steady states at the different time, will give you the history and will allow you to collect the history of your field evolving through time and allowing you to extract the unsteady phenomenon features (e.g the oscillation frequency and the Strouhal Number).
Not to forget, another method to analyse the vortex shedding is to decompose what you obtain from the steady flow simulation in different mode, hence to do a singular value decomposition of your field (very similar to the POD technique).


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How high could modern transonic commercial transport designers reasonably push L/D ratios?
One facet of the evolution of commercial transonic transport design since 1970 years is that L/D ratios (alternatively, the M * L/D) have increased only mildly over time. For example, see the following photo. I always found this fact to be incredibly surprising, because over the same time period the capabilities of CFD techniques to optimize airfoils and wings has increased dramatically. I expected L/D ratios to be much higher today than they were in the late 1960s.
The reason I've seen given which reconciles the simultaneous lack of significant advance in L/D ratios (a measure of aerodynamic efficiency) and immense increase in CFD capabilities (which should increase aerodynamic efficiency) is that designers often "trade" improvements in aerodynamic efficiency for reductions in structural weight. Firstly, I'd ask, is this a correct explanation for the only modest improvement in L/D ratios seen over the past 25+ years?
Note: one example that's given of this is that a reduction wave drag achieved by reducing shockwave generation in transonic flow can allow aircraft to fly faster, or fly the same speed at greater wing thickness, and the latter is often chosen to reduce structural weight.
Secondly, this question got me thinking, if the above explanation is correct, are there any references that attempt to model what the "counterfactual" L/D frontier would be for transonic transports if aerodynamic efficiency was not traded for weight improvements? That is, how high could we reasonably push modern L/D ratios for commercial transports if that was our main concern?

Progress isn't linear, and there's little reason to believe that an advance in a hardware or software design tools would correlate directly to a proportional advance in the end item.  Since the beginning of the jet age airliners have been round tubes with swept wings.  Improvements on this basic form have been incremental from there.  From my perspective it's like asking why architects can't design better houses using CAD than with paper and pencil.  Just an observation FWIW...

First of all, I am sceptical regarding your graph. The Fokker F-28 or the Short 360 are way too good while the Boeing 767 looks poorer than what other sources claim. Size alone helps already by making the viscous effects relatively smaller.
Next, despite L/D featuring linearly in the Breguet equation, airliners are not optimized for best L/D but for lowest seat mile cost. This means they fly faster that what achieving a high L/D would require in order to increase their transport efficiency. If we only reduce cruise Mach, the optimization will lead to less sweep and higher aspect ratios like in the Boeing SUGAR studies.
A quick search did not reveal L/D figures for such airliners, but with their braced wings of an aspect ratio of 27 and only 20° sweep I would estimate their L/D to be in the high Twenties if not even low Thirties. Note that some designs use a third engine at the end of the fuselage to ingest its boundary layer in order to reduce pressure drag.
Another avenue is active laminarization by sucking away the boundary layer. Early studies by Erich Pfenniger and others at NASA (PDF!) claimed L/D improvements in excess of 20% and a NASA LaRC study on the basis of test data obtained with a modified Boeing 757 concluded that a 15% reduction in block fuel consumption is possible with laminarization over the forward 50% of the wing of a 300 seat airliner.

L/D gains with different laminarization techniques (source). NLF = Natural Laminar Flow by shaping the pressure distribution, HLFC = Hybrid Laminar Flow Control mixes in some sucking, LFC = Laminar Flow Control by sucking at several chord lines.
This would bring a modern design like the A350 or the Boeing 787, which by themselves already have an L/D of close to 20, to an L/D in the mid-twenties if the active laminarization does not add weight or consume additional energy. Since both assumptions are rather unrealistic, and forty years later we have yet to see any commercial application, I expect that the effort for active laminarization eats up most of its benefits.
To answer your question I need to make assumptions how far you want to move from the current operating point of airliners. If speed should stay the same, we have already reached almost all of the reasonable potential and increases come from lower fuel burn of engines which allow to reduce wing chord a bit.
If, however, flight speed can be reduced by 0.1 Mach, large gains are still possible and I expect L/D values to become 50% higher if economics demands the highest L/D possible.


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Progress in supercritical airfoils - an answer
I previously asked a question about progress in improving the characteristics of supercritical airfoils. The basic supercritical airfoil design was created in the early 1970s by Richard Whitcomb. I was curious in understanding whether the airfoils of ~1990 and beyond, because they were designed using much more advanced CFD tools than Whitcomb's sections were designed with, were improved in performance, despite embodying the basic supercritical ideas (which were, generate a region of supersonic lift, weaken the shock wave, move it to the rear, generate lift from the aft using increased camber).
I found an interesting set of images that I thought the community would appreciate which speaks to my question.

You can clearly see how, in tandem with increasing complexity of CFD codes, the pressure distributions associated with the outboard sections of the wings became more aggressive. The shocks became weaker and they were pushed from 40% of chord to around 55% of chord.

You should add this as an answer to your original question. Answering your own questions is perfectly fine.

I'm going to VTC as a duplicate of your previous question so it will point over there. Pilothead is right that you should make this an answer to the other question.

Would you please share the source of those images?

As I understood the really big improvement for the later generation of supercritical airfoils was to tame the stall behaviour.  The early ones suffer from vicious leading edge stall due to a laminar separation bubble that forms.  The aerodynamic stall starts at the LE and spreads to the entire wing immediately. Airplanes typically require stick pushers to preclude aerodynamic stall from occurring.


text:
Interpreting a graph depicting airfoil technology improvement

I found this image in a paper titled, "Design and validation of advanced transonic wings using CFD and very high Reynolds number wind tunnel testing" that was published in part by Mark I. Goldhammer, the former Chief Aerodynamicist for the Boeing 777 project. In it he describes the evolution of wing sections over 4 decades. He notes the core CFD tools used to design each "generation" of wing, including:

NACA empirically-derived sections for the 1960s generation
Subsonic panel methods for the 1970s generation (in which he includes early widebodies)
2D Transonic Viscous Methods for the early 1980s generation
"Early 3D transonic methods..primarily small disturbance methods that lack reliable viscous modeling" for the late 1980s generation

However, I struggle to understand what this graph is really depicting. While I understand that each iterative CFD method mentioned above is "advancing" the airfoil in some way, it's not clear to me in what way the airfoil is more advanced and how that is represented in the airfoil geometry (either of a single section or a set of airfoil sections). Would anyone be able to help decipher this graph?

I don't think this graph is supposed to be very quantitative, otherwise there would be a "real" scale on both axes instead of the two-ended arrows and the hatched lines would be actual curves supported by some data points for specific airfoils.
What he's saying is that for a given airfoil thickness (constant y), the critical Mach number could be improved from one decade to the next, allowing faster flights without the drag penalty of transsonic flow. Or, for a given critical Mach number (constant x), the airfoil could be thicker, so the wing could be lighter or longer and more aerodynamically efficient. Or a combination of the two.

Yup, I guess I was just wondering what changed about the underlying airfoil or sets of airfoils to allow that improvement from one decade to the next. He mentions the specific underlying CFD methods that allowed for the "jump" from one line to the next, but not what the output of those methods were. For example, I was hoping for a sentence like: "because of the use of 2D viscous solvers, we learned that varying the t/c ratio across span in XYZ pattern led to less drag penalty" or something like that.


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How have aerodynamicists improved transonic wing performance over time?
The following two graphs depict improvements in transonic wing technology that have occurred over the past 20-25 years. Both are sourced from Boeing Commercial Aircraft.
In essence, both note that over time, engineers have managed to improve the "airfoil technology level", allowing designers to make wings thicker and fly at the same speed or fly faster. This has been accomplished by pushing back the drag-divergence Mach number for airfoils.
However, I'm confused as to how this improvement occurred, which I think can be broken down into 2 sub-questions:

What new tools/methodologies were used in the design of each generation of wings?
What changes in the airfoil/wing geometry & pressure distribution changed to allow the higher performance?

Graph #1: Source:

Graph #2: Source: https://nap.nationalacademies.org/read/2035/chapter/9

Note - I posted a similar question to this earlier, but did not phrase it well. So this is partly an attempt to correct that.

"I posted a similar question to this earlier..." pretty much all your questions have been similar!

@MichaelHall not sure if this is a criticism or not, but I'm happy to take the question down if you feel it doesn't add to the forum. But yes, all my questions have been targeted to understanding how wing aerodynamic performance improved between the years 1975 - 2010. That is the aspect of the aviation industry I'm interested by at the moment.

And I think you'll find that as time has gone on, I've used the answers from awesome folks in this community to learn and ask progressively more detailed questions. So for example, I started off by asking about how sweep angles in late-1980s aircraft were reduced compared to earlier designs, which I was told was due supercritical airfoils. This question builds on that answer by intendeding to be a deep-dive into exactly how supercritical airfoils themselves have been refined.

Just a casual observation, it’s all good.  Equally happy to simply delete my comment.


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Why would a lower trailing edge angle allow for more rear-loading?
In reading a paper on transonic aerodynamics by Jeff Jupp (Chief Wing Designer for the A310), he states, "the trailing edge angle in modern airfoils is reducing significantly, almost to zero in modern airfoils. This has two effects, it makes the trailing edge pressure more negative....and helps increase the rear loading without increasing top surface pressure gradients. Picture included below.
Source
What I don't understand is why a zero trailing edge angle in a supercritical airfoil would allow you to increase the rear-loading. The rear-loading is typically seen as a function of the degree of aft-camber.
Thoughts from smarter people than me?

The following picture (slide 4 from this presentation) qualitatively explains what is stated by Jeff Jupp (and I suspect that a very similar picture is used also in his paper):

The airfoil at the top is a "standard" one, while the one on the bottom is a supercritical one. Let's concentrate on the $C_p$ plots for the upper surface at the right of the figure.
As usual, the $C_p$ increases from the leading edge till (more or less) the thicker part of the airfoil and afterwards decreases till the trailing edge. The decreasing part of the $C_p$ is the most critical because there the boundary layer becomes very susceptible to any external perturbation. And this is exactly what is seen in the upper plot for the standard airfoil: after a peak there's a sudden drop in $C_p$ corresponding to the detachment of the boundary layer (in this case the external perturbation causing the detachment is a shock wave).
In a supercritical airfoil the angle of the aft portion of the airfoil "is reduced significantly, almost to zero" in order to smooth as much as possible out the decrease of $C_p$: its variation is now "spread" over a bigger portion of the upper aft surface, as visible for the supercritical airfoil at the bottom. That the $C_p$ remains more constant and higher on a bigger portion on the aft airfoil "increases the rear loading without increasing top surface pressure gradients".
This spreading of the aerodynamic loading toward the back of the airfoil has some important consequences, some of them also highlighted by Jeff Jupp like, on the negative side, the higher "weight of the flaps and the other secondary structure at the rear of the wing"; but, on the positive side, retarding as much as possible the drop of the pressure coefficient retards both the detachment of the boundary layer and the generation of a strong shock wave at transonic speeds, which is why supercritical airfoils have been introduced in the first place.

Two possibilities of "trailing edge angle" would be sweep angle and reflex angle.
Reduction of sweep angle was considered as here.  Reducing sweep near the wing tip could help increase rear loading.
But more likely the trailing edge angle is in the vertical.  Reducing the trailing edge angle of a reflexed airfoil to zero would account for making trailing edge pressure more negative (upper wing) and increase rear loading.
A 3 dimensional picture of the wing in question would be very helpful.


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What technical features account for the P-51B/D's low drag relative to other fighters of the day (e.g, Spitfire Mk IX)?
I am really sorry to dredge a popular topic up again. Unfortunately, after reading through some papers on the P-51 as well as all the past forum threads I could find on this topic, I have found myself very confused about the source of the P-51(B or D)'s drag advantages over the Spitfire Mk IX.
First, let's establish some facts.

In flight tests with equivalent engines & power-plants, the P-51B/D was 25-30 MPH faster than the Spitfire in level flight.
Drawing from David Lednicer's (a professional aerodynamicist's) comparison of WW2 fighters (Source: http://www.wwiiaircraftperformance.org/mustang/Lednicer_Fighter_Aerodynamics.pdf), he states "with the same version of the Rolls-Royce Merlin engine and propeller installed, the Mustang X was measured to be 23 mph faster than the Spitfire IX....The P-51B is even faster than the Spitfire."

The Mustang had lower total drag than the Spitfre. Again, showing a table from Lednicer's article, which uses drag estimates from flight tests and wind tunnels of the era. (Note: he notes in the article that he has been conservatively high with the P-51's drag).

So we know that the Mustang was faster than the Spitfire and had lower total drag.
However, I have been confused out of my mind as to what technical features account for this lower drag. Can this community come to the rescue?
Now, the most obvious answer to this question would be that the P-51B/D incorporated an airfoil section that was designed to maintain larger regions of laminar flow on it, lowering the skin-friction drag of the wing. However, to quote Lednicer again: "in service the aircraft was unlikely to have a substantial laminar flow on the wing and transition occurred in the first 15% of the chord".
Additionally, to quote John Anderson:

So it's unlikely that significantly reduced skin friction drag on the wing accounted for the lower drag unless someone has information that shows that whatever <15% extent laminar flow on the P-51B/D wings was markedly higher than similar extent of laminar flow on the wings of the Spitfire IX or other fighters of the era.

The P-51 was one of the first, if not the first, airplane design in which the aerodynamics of its cooling system were focused on- and in which the airflow through the radiator and its ducting were given the same sort of attention as the airflow around the fuselage.
Indeed, the origins of the P-51 design were originally investigated in the context of an aerodynamic cleanup of the P-40- in particular, reducing the drag associated with the P-40's radiator placement and cooling air flow through it.
This led to the fundamental idea of burying the entire radiator within the fuselage profile to reduce drag and using an advanced scoop design to bring airflow up into it, and using variable exit doors to control the airflow through it.

The above answer by Niels is on the money. According to an article I read recently by one of the actual designers of the P51, the reduction in cooling drag was the main reason for the better speed. It is late and I don’t have time to search for it, or the man’s name but I am sure he was quite clear on that and had data to back up what he said. The laminar flow wing had a lot less to do with it than I was led to previously believe according to him. The flow over the wing was not as ‘laminar’ as the designers thought it would be, but the radiator design was very, very good.


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How do you know what altitude to be at for "APPR" mode to work?
How would you know what altitude to be at to capture the glideslope on an autopilot RNAV or ILS approach? (I am mainly focused on MSFS airliners, but this could apply to other planes or in real life)
When you are too high, the autopilot does not descend for the final. It just flies straight over the runway and disables approach mode. Sometimes, you set the autopilot's altitude to 3,000 feet, and it works. Sometimes, not. This can be annoying, because you then think your autopilot has a problem. Well, how do you fix this?

Yes. That's what I meant. When you are too high, it will still fly the approach, as if it was going to descend, but you have to be below the altitude constraint for the waypoint to actually capture the GS. So basically it just flies above the runway and does not descent at all.

Is this a home flight simulator question? In real instrument flights, the approach chart has the crossing altitude for each waypoint and the connecting segment will never have a slope greater than the final glideslope so the only physical option is to intercept the final slope "from below"

The best way to find the platform altitude, is to look at the respective approach chart. For example, if you want to fly the ILS approach into runway 09L at London Heathrow, you can go to the UK AIP, navigate to "Part 3 AD 2 Aerodromes", select London Heathrow (EGLL), go to section AD 2.24 and open the chart for INSTRUMENT APPROACH CHART ILS/DME I-AA RWY 09L - ICAO:

I marked the platform altitude in red: 2500 ft AMSL. So you should be at 2500 ft when passing DME 7.5 on the localizer to intercept the glideslope.
Note that ATC might clear you to descend to a lower altitude. For example, you might get vectored onto the localizer closer to the runway and ATC could therefore clear you to e.g. 2000 ft instead. In absence of such an instruction, you should maintain 2500 ft until intercepting the glideslope.

Yeah but on simulators the charts don't work (I have tried using one and it didn't capture the GS) and ATC sucks. The best way for SIMs is the other solution, but yours is for sure the better one for IRL.

@DaCuteRaccoon Yeah, sim ATC is useless, I agree, unless you try VATSIM, IVAO, etc. But flying the approach according to the chart should work exactly like in real life, as long as your sim is realistic enough...

@DaCuteRaccoon So go purchase a set of instrument procedure charts. In the USA a paper book for a large region of the USA is $8 at any small-airport pilot shop or they can be ordered online. Or you can get them free on the FAA website (and a "chart users guide") skyvector.com is a graphical way to select airports and then get a list of proceedures. Low level IFR en-route charts are helpful for getting to an initial approach fix. In other countries or if you just prefer the format, you may need or want Jeppesen charts. There are tablet apps paid or free that can be used to manage all this.

Oh and charts update every month(for legal use when flying) but for simulator practice, about 95% of the data doesn't change in a year and far fewer changes in a month. The bigger problem will be the simulator being based on old data and approaches that have been removed from the current publications.

When you learned how to do an ILS approach, they probably said to fly lower than the glideslope. Usually, this is somewhere around 2,000 feet. But it can vary. A solution could be setting the autopilot altitude to the lowest setting, but, well, you're just not supposed to fly for 10NM at 100 ft agl.
The most common solution is to view an aeronautical chart of the area. You can see the altitude, there, but it's hard and I don't recommend this. I don't even know how to do it, as I have never had a problem that needed that (I use the sim, but if you fly IRL, please look at the charts).
The solution for SIM users, is a button. On some planes, there is a button labeled "CSTR." That shows constraints. Little purple numbers will appear on your map if you have configured your approach. On every waypoint, there will be one. If it has a line under it, e.g. "2000" that means you have to be 2000 feet OR ABOVE. No lower than 2000 feet. If it has none, it means you have to be below that. Usually it's the waypoint right before the runway. If it says "1700", then after you pass the one right before that and are headed for the "1700" one, then set your target altitude to 1700, and THEN turn on approach mode.

Hint: CSTR is Airbus specific; on a Boeing, press the DATA button to see the constraints. You can also view them on the (M)CDU flight plan page (shown on the right next to each waypoint).

@Bianfable I'm not familiar with a "DATA" button on the 737 MCDU. Is that what some Boeings use in place of the "LEGS" page / button? Or something else specific to other Boeing jets?

I don't think "DATA" is on the 747 MCDU. I am pretty sure, well I can send a picture to where it is on my Airbus.

https://paste.pics/PDS8F

@RalphJ I meant the DATA button on the EFIS controller, which is AFAIK similar to the Airbus CSTR button to show times and constraints on the ND next to each waypoint.

@Bianfable I'm following you now... yes that button on the EFIS control does exactly that. I don't use it much because, to my eye, it clutters up the display (and I typically go to the LEGS page for altitude constraints), but that also shows them, on the map. Thanks for the reminder!

Oh LOL sorry I don't really use the 747, mostly just a320neo, tbm930, and all the cessnas.


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Why does the INERT SEP take so long to deploy?
I have experience in Microsoft Flight Simulator with the Daher TBM 930. It has an inertial separator, which I use for de-icing in the simulator. I watched a video on it, and I understand how it works, but don't get why it takes 40 seconds to deploy. It's pretty simple, only a valve opening in the turbofan and another thing coming down to redirect particles into the valve.
So why does such a simple mechanism take such a long time to deploy? Maybe I am not fully understanding how it works?

Well you can get it straight from the horse’s mouth…

Due to the strong aerodynamic loads on the movable doors, the inertial separator uses an electrically driven screw jack to open or close the inertial separator.  In addition, fast movement of the IPS doors can cause severe disruption of the air flow to the engine, potentially causing a flame out or compressor stall.  Therefore actuation is delayed and takes place gradually. It takes a period of time – approximately 40 seconds, for the IPS to move through its full range of motion.

Yeah, that's the video I watched. But thanks for your answer, now I get it!


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Can supersonic booms intensify as you fly longer?
If someone is flying just above Mach 1 for 10 minutes, I would think the sound/shock waves would just pile up and gain more energy, resulting in a very intense and loud boom when the pilot slows down.
However, I don't think this is the case. I saw one answer online, that said it would not, but it did not explain why. Even if it doesn't happen, I don't understand how ten seconds of sound waves behind you piling up would be the same shock intensity as ten minutes of supersonic flying.

I'm imagining a boat, cruising across a lake, with a progressively larger wake until there is dry land behind the boat. Maybe that's what Moses did?

@tedder42 No, Moses was never reported to have gotten into the water prior to it parting ;)

Unfortunately air and the sound waves moving in it are invisible. But luckily in nature waves exist that can be actually easily seen. These are the waves generated by perturbations at the interface between a liquid and a gas: the waves generated by a duck swimming in a lake  I think that watching those waves generated by a duck or a boat is the best way to understand Mach cone and other supersonic concepts

Well, actually, you can see the transonic vapor cone. It now makes sense to me. Thank you.

@mins I prefer this one. Anyway those don't really help understanding the concept of Mach cone or shock waves since that vapour condensation is visible only around Mach 1 but the sonic boom is heard at any speed bigger than Mach 1 (sorry for having deleted my previous comments but I found a mistake in it )

@DaCuteRaccoon: jokes aside, in that picture of the duck you can truly see a Mach cone which is formed through the coalesce of the sound waves when the speed of sound is reached and exceeded. If you stand on the shore, the moment when that front of waves reach you is when you'd hear the sonic boom.

@tedder42 And now I can't get the image of Charlton Heston, playing George Washington, crossing the Delaware River in a speedboat going Mach 9 out of my head.

They don't really pile up like that.  The 'pile up' description of how shock waves form is pretty flawed.
The shock wave forms at an angle off of the body.  Generally speaking, it forms a cone off of the nose of the body.  Technically, there are usually two cones, one from the nose, the other from the tail caused by the air coming back together again.
As the aircraft flies forward, the cones sweep out a footprint on the ground.  When the cones pass over you, you hear the boom.
The aircraft's lift contributes to the strength of the boom.  So, the boom will actually be slightly weaker at the end of the mission (because a bunch of fuel has been burned off).
When the aircraft turns, it can create regions of a stronger and weaker shock.  First, the lift is increased during a turn.  Second, the turn causes the shock to spread out (on the outside) and squeeze together (inside of the turn).  This causes some boom focusing and can strengthen the boom.
The atmospheric conditions between the boom causing aircraft and the ground can have an effect -- winds, temperature, humidity, etc. can change how the boom propagates.  An aircraft could take off in conditions that dissipate the boom, but at the end of the mission could be somewhere where the boom propagates with fewer loses -- and would therefore be perceived stronger.
When an aircraft slows down at the end of a mission, the strength of the boom changes and this can also cause some focusing like a turn.  Similarly, if the aircraft dives or climbs, it can cause focusing or spreading of the boom.
So while the shock waves don't pile up in the sense of accumulating throughout the mission, they can interact in complex ways to affect the strength of the shock from the aircraft.
Engineers working on low boom aircraft not only study the shape of the aircraft, but also the propagation of the boom to the ground and the trajectory the aircraft will follow.  It may be that turns are planned to focus the boom in unpopulated areas, or deceleration / descent at the end of missions is done offshore before coming over land...  There are many interesting challenges to be considered.

What about altitude? I used to hear that flying higher didn't help because it just spread the boom farther out but that it was just as loud. Which always seemed counterintuitive because there is a fixed amount of energy. I have since heard that altitude  does decrease the boom somewhat. I also heard that the boom is much  louder directly underneath the flight path, and less jarring if farther away. Which would explain people in the UK who said they could hear the sonic boom from Concorde when it was approaching the coastline and that it wasn't that bad.

Trust your intuition.  The same way any sound grows faint the further you are from it, sonic boom effects are less the higher the airplane is.

@StevePemberton: Perhaps that's a distortion of some actual facts: It wasn't possible to fly high enough for the boom not to be a problem directly under the flight path (at least for Concorde).  And more people could hear the boom at all if flying higher, even though the sound power was more spread out for all of them.

@PeterCordes - that sounds like a likely explanation, it's certainly not unusual for the simplification of a complex topic in public arenas, which is needed, to go too far into oversimplification, leading to misconceptions that then take on a life of their own. Another example is the common misconception that a sonic boom is created only at the moment when an aircraft exceeds the speed of sound. Somewhat understandable since for many years the phrase commonly heard was "breaking the sound barrier". With no other explanation this can give an incomplete picture of what is actually going on.

@MichaelHall - to some extent yes. But when it comes to fluid dynamics I try not to rely too much on intuition. While the things being said were counterintuitive, I held off dismissing them until I got some confirmation that things worked a little more as expected in this regards. As Rob McDonald's answer illustrates, there are many factors that can affect the strength of a sonic boom for a particular location at any one moment. I'm interested in quantification of the effects of altitude and distance which is somewhat hard to come by. Which is understandable because of the many variables.

Very well, I cannot math out the fluid dynamics of it, but the "boom" is ultimately a sound, and we all know how sounds disburse and weaken with distance as the wave spreads.  Just like a near vs far clap of thunder.  Adding to that I have heard sonic booms both far off and very close by, so I know firsthand that they are no different.

@StevePemberton Altitude certainly makes a difference -- in particular the temperature layers of the atmosphere the boom must go through.  If you are only slightly supersonic -- say Mach 1.1 at altitude, where the air is very cold, by the time the shock wave reaches sea level, that speed might not be supersonic anymore and the boom will actually go away.  Yes, the groundtrack boom is stronger than the off-groundtrack (unless there is focusing from turning, etc).

@MichaelHall - that makes sense, however complicated the generation of the shock waves are, once produced they will behave like other sound waves. Even though the complications of the production as well as variables in the atmosphere make definitive predictions difficult or impossible, the general trend would be there regarding altitude and distance. In a few years I think we'll have more experience with this when Mach 1.2 business jets begin flying with reduced sonic booms. Anyone can then hear a sonic boom and then go onto Flightradar24 and see where it is and how high it is.

Even if it doesn't happen, I don't understand how ten seconds of sound waves behind you piling up would be the same shock intensity as ten minutes of supersonic flying.

Sound is pressure, and sound is energy. If someone hears the sonic boom, that's because energy from the shockwave is leaving the vicinity of the airplane, and reaching the ground. If it were to "pile up" for 10 minutes and then suddenly all be released, BOOM, that would mean that nobody would hear anything for the first 10 minutes, and obviously that isn't what happens.
So, the "piling up" explanation is really pretty misleading. What it really means to be supersonic is that pressure waves can't travel forward relative to the airplane, only backward. Since pressure waves can't travel forward, there can't be a gradual change in pressure, or a gradual redirection of air around the body. Instead, it happens suddenly, in a very short distance, which is the "piling up" that forms the shockwave. But the shockwave still moves away from the airplane: up, down, to the sides, and especially backwards. When it reaches an observer, they hear a boom.
The airplane is constantly feeding energy into the shockwave by disturbing the air (and it gets that energy from the thrust provided by its engines), and the shockwave is constantly carrying energy away. In general, the two processes are in equilibrium — the shockwave is carrying away just as much energy as the airplane is putting in. There's no continuous buildup.

I think the "pile up" analogy would be fine if one couples it with an image of trying to dump a continuous stream of sand onto a tower's flat roof.  After awhile, the amount of sand on the roof will stabilize at some limit where the rate at which sand flows off the roof has increased to match the rate at which it is applied.

So, the "piling up" explanation is really pretty misleading. What it really means to be supersonic is that pressure waves can't travel forward relative to the airplane, only backward.

This does not mean air directly in front of the nose would travel backwards relative to the plane.  Then the air would be in the nose and the nose would be broken.

There's no continuous buildup.

There is a buildup.  It's just not a buildup of sonic boom.  The skins of supersonic aircraft get hot and must be cooled to remove the buildup of heat.  The cooling could be infrared radiation.  In the case of the SR-71, the heat was transferred to the fuel.  When the aircraft slows down, some of the heat stored in the skin is released.
These details may help explain the sonic boom better, in addition to the other answers:

Compressing air stores energy in both compression and heat.  Some of the heat is transferred to the plane.
Air can travel faster than the speed of sound.  Molecules that have just been hit by a supersonic aircraft are very likely to be going faster than the speed of sound.  Their motion is partly random.  The motion of the molecules relative to the plane prevents them from accumulating on the front of the plane.  As mentioned by hobbs, the shockwave moves to the sides.
If you are on the ground listening to a passing supersonic aircraft, you hear a boom.  The sound waves produced by the aircraft long before it passes are weak.  The sound waves produced by the aircraft long after it passes have not arrived yet when you hear the boom.  As a result, the boom you hear from the ground mostly comes from a brief period of flight.

When the cones pass over you, you hear the boom.

Another way to say this is that the boom exists before and after you hear it, but you only hear it when it is at your location.

The total energy in the shock cone does increase the longer you fly.  However, since the cone gets bigger, the energy is spread out more.  It will "intensify" globally, but local measurements of loudness will not increase.

Fluids are more complicated than line drawings.

Re "The total energy in the shock cone does increase the longer you fly." -- really?  I'm more inclined to believe last paragraph of this answer -- https://aviation.stackexchange.com/a/101084/34686

@quietflyer I agree with the last paragraph of that answer, so long as it refers to the energy density.  There is no contradiction.


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Does dynamic stability decrease with airspeed
Context: I am writing my thesis on the topic of satellite drag reduction by the means of near-specular surface scattering. The theme touches aerodynamics in some sense, but the physics are honestly quite unrelated.
I do however have a subsection where I discuss the matters of static and dynamic stability. Tldr: static stability can exist, but dynamic can not, or it would be absolutely vanishing.
I seem to recall from somewhere that dynamic stability also diminishes as airspeed increases in atmospheric aerodynamics, and had an off the cuff statement to that effect. I could not find a source later on, and removed the statement.
So: Is it true that dynamic stability diminishes as airspeed increases? The reason would be that the aerodynamic force vector becomes more and more aligned with airspeed vector, and the perpendicular damping forces shrink. Would there be a peer reviewed source for this?
(obviously, while I have worked in aerospace a while, I am not an aerodynamicist.)

Is it true that dynamic stability diminishes as airspeed increases?

Yes, when the solid body eigenmodes are concerned.
In aerodynamics, dynamic stability is mainly achieved in two ways:

Secondary motions (eigenmodes, flutter) induce velocities and forces which can either propel (= unstable) or dampen (= stable) those secondary motions.
Drag helps to let those motions die down over time.

When flying fast, the induced speeds become smaller relative to the primary motion of the airplane, so the induced forces also become smaller. This is evident in flight at high altitude, when eigenmodes like the phygoid or the dutch roll become more intense.
Your expression of the alignment of the aerodynamic force vector with the airspeed vector and the shrinking of perpendicular forces says the same, only in other words.
What also reduces damping is airframe elasticity: With the higher dynamic pressure at high speed, the induced forces cause deformations which in turn reduce these secondary forces. Again, oscillations take longer to die down. Undamped oscillations become much more dangerous: Flutter will cause almost instantaneous structural failure at high speed when the same flutter mode at low speed might still be non-destructive, with limited amplitude.
Flutter is a topic of its own: Here, you need frequency neighbourhood between a structural eigenfrequency and the frequency of aerodynamic oscillations which is proportional to true airspeed. First, increasing speed will increase damping (think of a mass-spring system, with the spring increasing in stiffness as dynamic pressure is increased), but as the aerodynamic frequency grows with speed close to the structural frequency, both mutually increase and damping rapidly becomes negative.
Any decent book on flight dynamics should cover this topic - sorry for not giving a discrete page and citation!


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What is the mode of operation of a Hobb's meter?
We have a Hobb's Quartz meter that is connected to a power supply test station that reads the amperage and watts.  It only indicates a reading on the test unit as the unit ticks every few seconds.
What is going on that causes this to only draw power while the unit clicks over the next time increment every few seconds rather than have a constant draw?

The "Quartz" labeling means that time is kept by a quartz crystal oscillator. The oscillator circuit does consume power, but it is a tiny amount (possibly as little as microamps) that your power meter is apparently unable to measure. The mechanical actuator (usually operated every tenth of an hour to advance the indicator mechanism) takes significantly more current, and you power meter is able to show that.
You may have assumed that the meter is more like an old-fashioned clock, moving at all times when powered, driven by a constant-speed motor, but this is not the case. Quartz crystals are significantly more accurate for time-keeping than motors.
If you had a power meter capable of registering sufficiently small amounts of current, you should be able to see the draw of the oscillator circuit when the device is connected (as opposed to zero when it is not connected).


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Can you help me to identify the aircraft in a 1920s photograph?
Can anyone identify the aircraft in this photograph? The photograph was taken in Belgium in the summer of 1926. Does that say ‘Imperial Airways’? Thank you!

Somewhat out of focus but another image of the same aircraft.
https://digital.lib.buffalo.edu/items/show/9240

I believe this is a Armstrong Whitworth AW154 Argosy.

(photo source, CC-BY-SA 3.0)
The taper on the engine pod looks correct, it appears to be similar dimensions to the photo in the question.
Per BAE they were used for passenger service UK-Paris beginning in 1926, and Brussels is mentioned:

The AW154 Argosy was used on routes to Paris, Basle, Brussels and Cologne

I thought it might've been a DH.66, but its engines are mounted to the lower wing.

I believe it is an Armstrong Whitworth Argosy, specifically the MK.I as that was in service with Imperial Airways in 1926:

This is based on the shape of the rear door, the two large main wheels, the position of the registration relative to the door, the styling of the windows, the uncowled wing-mounted engines, and the straight lower fuselage.


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Coordinate system for describing 3D target position relative to plane FRD
I'm trying to understand what is the most popular convention for describing the position of a point in 3D space relative to a plane's body frame, with axis "front", "right" and "down" (FRD).
In mathematics I know that it's popular to use the spherical coordinate system (polar angle, azimuthal angle, distance), but the "polar angle" seems to be inconvenient (a point in front of the plane would have a polar angle of pi/2). Is the elevation angle more often used instead of the polar angle?
The tuple (elevation angle, polar angle, distance) have the nice property that a target in front of the plane has angles 0 and 0. But assuming that the Z axis is "down", the name "elevation angle" seems to be misleading (when the Z axis is "down", a positive angle would mean "down", which doesn't seem to describe "elevation"). Therefore this approach also doesn't seem "right" (although mostly because of the name "elevation").
What is the most practical and intuitive coordinate system used in aviation for describing a 3D point position?

For what purpose, and by whom?

A positive elevation angle (positive rotation around the Y axis) is up, not down. Apply the right hand rule of rotation (stretched out thumb, curled fist) to the Y axis.

"Popular" depends on your target audience and how they use that information. If your audience is a developper team for flight simulation, the "Z down axis" is an endless debate and has no conclusive preference; it all comes down to 3D transformation matrices so everyone just cope with whatever data comes. For instance, PBA locally known as Pitch/Bank/Angle assumes Y pointing up and Z pointing right wing or the other way around. Then various tools usually uses different reference models, transformations is most likely always the answer. I second Michael : purpose, by whom, and for whom ?

The most common, from a pilot's experience, is clock position, range, and relative altitude: "Traffic at 2 o'clock and 9 miles, 1,000' above your altitude."
Everything else is easily converted to whatever reference frame best facilitates the needed computations or is most intelligible to the audience, whether that is an X-Y-Z coordinate system, or angles, or whatever else.